A drivetrain of a motor vehicle of the above mentioned kind has been disclosed in DE 199 34 696 A1 and DE 101 52 471 A1. In this known drivetrain, the summarizing gear is, in each case, made as a simple planetary gear with one sun gear, one planet carrier with several planetary gears and one ring gear. The ring gear forms the first input element and is rotatably fixed to the crankshaft of the internal combustion engine. The sun gear forms the second input element and is rotatably coupled with the rotor of the electric motor/generator. The planet carrier forms the output element and is rotatably connected with the input shaft of the shift transmission. The lock-up clutch is located between the sun gear and the planet carrier of the planetary transmission.
In the drivetrain, according to DE 199 34 696 A1, the lock-up clutch, unlike the construction assumed here, is designed as a dog clutch so that the lock-up clutch can be engaged only during synchronous running of the internal combustion engine and of the input shaft of the shift transmission thus being utilizable only with limitations. To make driving the motor vehicle only with the electric motor/generator possible, a directional free wheel is disposed between the crankshaft and a housing part whereby the crankshaft is protected against a reverse turn and thus the input torque of the electric motor/generator is supported against the housing. To make starting the internal combustion engine possible while the motor vehicle is stationary, one other directional free wheel is disposed between the input shaft of the shift transmission and a housing part whereby the input shaft is protected against a reverse turn and thus the input torque of the electric motor/generator is supported against the housing.
In the drivetrain, according to DE 101 52 471 A1, the lock-up clutch, such as assumed for the instant invention, is designed as a friction clutch so that the lock-up clutch can also be used in case of a speed difference between the input shaft of the shift transmission and the internal combustion engine for transmitting a torque in the slip operation. When the motor vehicle is stationary and the transmission shifted to idling speed, to make an impulse start of the internal combustion engine with the electric motor/generator possible, one other friction clutch is situated between the input shaft of the shift transmission and a housing part whereby the input shaft, after reaching a starting rotational speed of the electric motor/generator can be decelerated for starting the internal combustion engine.
In the description of the invention and without limiting the scope of protection, herebelow the point of departure by way of example is an extensively identical construction of the drivetrain wherein the lock-up clutch is assumed as a friction clutch, particularly as a wet multi-disc clutch, but alternatively can also be designed as a dry clutch. Alternative to the known arrangement, the lock-up clutch can also be situated between the ring gear and the sun gear, that is, between the crankshaft of the internal combustion engine and the rotor of the electric motor/generator.
In the normal driving operation, the lock-up clutch is completely engaged so that the planetary transmission is blocked and rigidly rotates. In this state of operation, the rotational speeds and the direction of rotation of the internal combustion engine of the electric motor/generator and of the input shaft of the shift transmission are identical. In this state, the electric motor/generator is operated predominantly as generator for supplying the electric vehicle circuit, but in certain operating situations, especially in acceleration phases of the motor vehicle, it can also be operated as a motor.
Upon appearance of a gearshift requirement which, according to operating parameters of a control device, can be triggered automatically or by the driver manually actuating a shift control element, such as a shift lever in a manual shifting gate or a shift paddle disposed behind the steering wheel, a drivetrain of this sort, immediately prior to the gearshift has to be made extensively torque free within the shift transmission. Moreover, the lock-up clutch has to be entirely open in order to make possible within the summarizing gear a rotational speed compensation determined by the speed ratio change. It is altogether achieved hereby to make possible that the activated load gear be torque free de-activated and the target speed to be activated be load-free synchronized and subsequently activated. The load freedom in the shifting and synchronizing elements concerned makes low shifting forces possible, slight wear on the shift clutches and on the synchronizing elements, short shifting times and great shifting comfort.
Hitherto it had been usual for this purpose to reduce to zero the torque of the internal combustion engine and of the electric motor/generator independently of each other and to open the lock-up clutch without coordination therewith. As a result of the relatively quick response behavior of the electric motor/generator and of the lock-up clutch, the same as the relatively slow response behavior of the internal combustion engine, this can lead to undesired compensation motions within the summarizing gear which can lead to inadmissibly high rotational speeds and high loads produced by inertia.
In a design of the summarizing gear as planetary transmission in the known arrangement according to DE 101 52 471 A1, in this situation, it is thus possible that a quickly load-free, shifted electric motor/generator and a prematurely disengaged lock-up clutch lead to an acceleration of the internal combustion engine combined with an extremely high rotation of the electric motor/generator in opposite direction. Hereby damages can occur, both in the electric motor/generator and also in the planetary transmission. Moreover, the rotational speed differences caused at the same time in the planetary transmission make difficult and extend the subsequent synchronization of the target gear and the synchronization to be carried out after the gearshift between the internal combustion engine and the input shaft of the shift transmission.
With this background, the instant invention is based on the problem of indicating a method with which, in a drivetrain of the aforementioned kind, the torque reduction and the disengagement of the lock-up clutch, prior to a gearshift, can be carried out in the summarizing gear as quickly as possible without changes in rotational speed. Such a method is utilizable also in drivetrains containing similar components, but in different input coupling combination. It is to be taken into consideration at the same time that the internal combustion engine can be in the traction or coasting operation, the lock-up clutch can be from a completely disengaged state via a partly engaged to an entirely engaged (with overpressure) state and the electric motor/generator be precisely operated as generator or as motor or be shifted precisely without power.